1. Field of the Invention
This invention relates to methods and apparatus for synthesizing a single or multiphase AC output waveform of a substantially constant frequency from a multiphase input waveform of varying frequency. More particularly, it relates to control of the timing of the operation of the solid state switches which sequentially gate segments of each phase of the input waveform to the output as a function of the frequency of the input waveform.
2. Description of the Prior Art
In several applications, the production of a constant-frequency power output from a variable-frequency power source is required. One, and presently the most important, application in this category is aircraft power conversion. Here the prime source of electrical power is a rotating generator that receives its mechanical power input from the engine of the aircraft. Since the engine speed varies, usually over a 2 to 1 range, it is not possible for the generator to produce constant frequency output if coupled directly to the engine. Heretofore the general practice has been to insert a hydraulic constant-speed coupling device between the engine and the generator, thereby enabling the generator to be driven at a constant speed and hence to deliver a constant frequency power. Such a system has several disadvantages, not least of which is the relatively frequent and costly maintenance required.
An alternative system approach to aircraft power generation is to couple the generator directly to the aircraft engine, allowing it to produce a variable-frequency output power, as dictated by the engine speed. This variable frequency power is then converted to accurately regulated constant-frequency output power by means of a static frequency converter. This type of arrangement is generally referred to as a variable-speed-constant-frequency (VSCF) power generating system.
Two basic types of frequency converters have been proposed for VSCF applications. In one type of converter arrangement, the alternating voltage of the generator is converted first into a direct voltage by a (phase-controlled) rectifier circuit, then the direct voltage is converted back to alternating voltage (of the desired frequency) by a static inverter circuit. In the other type, a static frequency changer, which is capable of converting the variable-frequency alternating voltage of the generator directly into constant-frequency output voltage, is employed. The first type of arrangement is generally referred to as a DC link converter, while the second type is called a direct AC to AC frequency changer or frequency converter. Since the direct AC to AC frequency changer is capable of converting the variable-frequency generator power into constant-frequency output power in one stage, its operating efficiency is generally higher and its weight and size are usually lower, than those of its DC link type counterpart. For these reasons, the direct AC to AC frequency changer appears presently the best solution for VSCF power conversion.
Various types of direct AC to AC frequency changers have been proposed for aircraft VSCF applications. These include the naturally commutated cycloconverter (NCC), the unrestricted frequency changer (UFC), and the unity displacement factor frequency changer (UDFFC). For a detailed explanation of these frequency changers refer to pages 384 to 395 of the book "Static Power Frequency Changers" by L. Gyugyi and B. R. Pelly, John Wiley and Sons, Inc., 1976. These frequency changers have different operating and performance characteristics resulting in often mutually exclusive benefits and penalties when used in a VSCF power generating system. The major operating and performance features of these frequency changers in a VSCF power generating system can be summarized as follows.
The naturally commutated cycloconverter (NCC) employs controlled-rectifier type semiconductors (SCRs) with no intrinsic turn-off capability. These devices are commutated (turned-off) by the process of "natural commutation", by which the current is transferred without external forcing between the controlled-rectifier type circuit elements. This is achieved by proper selection of the switching instants relative to the instantaneous polarities of the input voltages, when the output voltage waveform is synthesized. Natural commutation is desirable because controlled-rectifier type devices are presently available with sufficiently high rating in small physical sizes. However, the restrictions in the output waveform construction to satisfy the conditions for natural commutation result in a lagging input power factor (at any load power factor) and in the generation of harmonic components in the output that are difficult to filter. The lagging power factor increases the rating and size of the generator; the harmonics necessitate a relatively large output filter.
The unrestricted frequency changer (UFC) requires switching devices with intrinsic turn-off capability (e.g., transistors) or an external commutating circuit. The generated output voltage waveform of the UFC is optimized for harmonic content; therefore, only a minimum amount of output filtering is needed. The phase angle of the current drawn from the generator is the negative of the load phase angle. Thus, a lagging load is seen by the generator as a leading load, and vice versa, a leading load is seen as a lagging load. In aircraft VSCF power systems, the load power factor is usually in the lagging (inductive) domain. Thus, the power factor seen by the generator is normally leading. This helps to keep the rating of the generator relatively low, close to the output rating of the UFC. However, at high generator speeds and under heavy inductive output loads, the generator may become overexcited. This may require undesirably high voltage rating for the semiconductors in the UFC or some form of external overvoltage protection. Another potential problem with the UFC is that at high generator frequencies, the switching rate of the semiconductors is rather high (that is, f.sub.switching =f.sub.generator +f.sub.out), which may result in undesirably high losses.
The Unity Displacement Factor Frequency Changer (UDFFC) requires two complete converter circuits with devices having an intrinsic turn-off capability (or an external commutating circuit). The two converters are operated in a complementary fashion so that the input displacement (power) factor remains unity under all output load conditions. Thus, the generator has to supply only the real load power demand. This results in a generator rating that is minimum for a given output rating. The distortion of the output waveform is low, and thus the filtering requirement is also relatively low. The switching rate of the devices in at least one of the converter circuits is the same as in the UFC, which may cause some concern for efficiency at high generator frequencies. The greatest disadvantage of the UDFCC is the requirement for two complete power circuits, which make it unattractive in most airborne applications, except possibly in those applications where the high output requirements would make device paralleling necessary.